The present invention relates to a semiconductor integrated circuit comprising an analog circuit including an operational amplifiers and a bias circuit for supplying a bias voltage to the analog circuit, particularly a semiconductor integrated circuit where the bias circuit includes a resistor.
Operational amplifiers and bias circuits used in conventional semiconductor integrated circuits are disclosed in, for example, Japanese Patent Laid-open Publication 219629/97. The bias circuit disclosed in the Japanese Patent Laid-open Publication 219629/97 is equipped with amplifying means for generating a bias voltage taking a reference voltage as an input while taking a resistor (hereinafter referred to as a reference resistor) as a load, differential amplifying means working in such a manner as to make an inputted reference voltage and the output voltage of the amplifying means the same, and current/voltage converter means for generating a bias voltage by converting current flowing through the reference resistor to a voltage. The operational amplifier is equipped with a transistor for constant current use, with the current values of the differential stage and the output stage being decided by supplying the bias voltage from the bias circuit to the gate electrode of a transistor for constant current use. Regarding the operational amplifier and the bias circuit, the current value of the operational amplifier is decided by the bias voltage only, with the bias voltage only being decided by the reference voltage and the reference resistor. The operational amplifier current is therefore constant and does not depend on the threshold values of the MOS transistors comprising the operational amplifiers and bypass circuit and fluctuations in the power supply voltage. The operation of the operational amplifier can therefore be made to be stable.
Related semiconductor integrated circuits consist of one or bias circuits of the above configuration and analog circuits. Load elements such as load resistors and driving means such as operational amplifiers of the above configuration for driving these load elements or constant current sources such as the aforementioned transistors for constant current use comprising the above operational amplifiers are formed at the analog circuit. The driving means and constant current sources comprising the analog circuitry both receive bias voltages supplied from the bias circuit.
There are also related semiconductor integrated circuits where analog circuits are divided into analog circuit blocks by gathering together constant current sources such as transistors for constant current use used in operational amplifiers capable of being simultaneously powered down, i.e. by gathering together current sources within the analog circuits from the point of view of power down control. Bias circuits are then provided for the respective analog circuit blocks with unnecessary analog circuit blocks then being selected and turned off. When powering down the analog circuit blocks in related semiconductor integrated circuits having analog circuit blocks, the output voltage of the corresponding bias circuits is switched over to, for example, zero volts and the current of the constant current sources within this analog circuit block is halted.
However, when driving means are formed on a semiconductor integrated circuit, a number of types of materials are used in making the load resistors driven by these driving means. The temperature characteristics of the load resistors and the manufacturing variations etc. are therefore different and the analog circuits therefore do not operate in a stable manner. The load resistors used here are resistors formed on the semiconductor integrated circuits or resistors (hereinafter referred to as "external resistors") externally attached to the semiconductor integrated circuit. Resistors used as reference resistors for the bias circuits are similarly resistors formed on the semiconductor integrated circuit or resistors (hereinafter referred to as "external reference resistors") externally attached to the semiconductor integrated circuit. For example, the case of using polycrystal silicon resistors (hereinafter referred to as "polycrystal silicon resistors") and diffusion resistors (impurity is diffused in a silicon substrate) as load resistors formed on the semiconductor integrated circuit and using metallic film resistors as external load resistors and then mixing these resistors on the same semiconductor integrated circuit has been considered. In this case, the bias circuit reference resistors are polycrystal silicon resistors, diffusion resistors or external resistors. In the following, load resistors and reference resistors comprising diffusion resistors are referred to as diffusion load resistors and diffusion reference resistors and load resistors and reference resistors comprising polycrystal silicon resistors are referred to as polycrystal silicon load resistors and polycrystal silicon reference resistors, respectively.
The metal film resistors can have a small temperature coefficient with a high degree of precision and it is therefore possible for external load resistors and external reference resistors to have small temperature coefficients with high degrees of precision. The structural variation between semiconductor integrated circuits for polycrystal silicon resistors is from 10 to 20 percent but the relative precision of resistance values occurring within the same semiconductor integrated circuit is good and the temperature coefficient is small. With diffusion resistors, the temperature coefficient is large and the resistance is low at low temperatures.
When the bias voltage of a bias circuit having a polycrystal silicon reference resistor is supplied to a driving means for driving a diffusion load resistor, the resistance value of the diffusion load resistor falls at the time of low temperatures but as the bias voltage does not change, the driving current is not increased and the outputted waveform therefore becomes distorted due to the driving performance of the driving means being insufficient. Distortion of the output waveform at the time of low temperatures similarly occurs when a bias voltage generated by the bias circuit employing the external reference resistor is supplied to the driving circuit for driving the diffusion load resistor. When the bias voltage of a bias circuit having a polycrystal silicon reference resistor is supplied to a driving means for driving an external load resistor, the resistor value of the polycrystal silicon reference resistor becomes larger than the designed value due to manufacturing variations. The bias voltage is therefore smaller than the design value, the driving current is small and the output waveform is distorted. In order to prevent these distorted waveforms, it is necessary to preset the driving current of the driving means for driving the diffusion load resistor and the driving means for driving the external load resistor to large values. The current consumed by the driving means as a result of this therefore becomes large. When the bias voltage of the bias circuit having a diffusion reference resistor is supplied to the driving means for driving the polycrystal silicon load resistor and the driving means for driving the external load resistor, the bias voltage becomes high at the time of low temperatures and the current consumed at the driving means becomes large.
In related semiconductor integrated circuits having analog circuit blocks gathered together from the point of view of power down control, the number of bypass circuits required is equal to the number of analog circuit blocks. The number of reference resistors on the semiconductor integrated circuit therefore increases and the semiconductor integrated circuit chip size is increased.